MORPHOLOGY OF CYCLOSTOMATA. 401 



the ventral portion of the pronephric Anlagen from the dorsal, 

 'which latter is differentiated into the tubules and constitutes the 

 dorsal portion of the pronephros. The ventral part of the An- 

 lagen separated from the dorsal retains anteriorly its connection 

 with the anterior most tubule and posteriorly with the segmental 

 duct. It is prolonged and bent out anteriorly in front of the 

 dorsal part. " Mollier's description is," Field says, " sub- 

 stantially in accord with my own observation," [loco cit., 



p. 286). This feature of the duct shows, it seems to me, a close 

 resemblance to the auteriormost section of the Teleostean segmental 

 duet which is, as above referred tu, bent in the same fashion. 



The segmental duct arises, according to previous writers, as 

 a longitudinal common furrow oft he parietal peritoneum, which 

 lurrow is later constricted off from the mother-layer and becomes 

 converted into a long canal. Molliee has observed the segmental 

 duct transformed directly from the mesoblast, just like the 

 glandular part of the pronephros, in the two somites behind the pro- 

 nephros. Whether the greater remaining part of the duct is formed 

 likewise by differentiation of the mesoblast, or by a backward 

 growth of the hind end of the duct iirst formed, he could not decide 

 with certainty ; but the observations of Field elucidate this point. 



"The segmental duct arises," Field says, "throughout its 

 entire length by a proliferation in situ of the somatopleure " 

 {loco cit., p. 223). The author has observed neither its epiblastic 

 origin nor a free growth of its posterior end, except in the cloacal 

 region Avhere it " grows across the cloaca free from adjacent tissue " 

 {loco cit., p. 223). In Stage v, the cloacal opening is seen. This 

 opening is found, in Fuuia and Bufo, in the vertical plane with 

 the middle of the twelfth somite, whereas it is below the twentieth 

 somite in Anihlyslonia (Field). 



